Adsorption is an important operation in many industrial processes. The effectiveness of an adsorbent depends, primarily, on its surface area, pore structure, and surface chemistry. The nature of the adsorbate which is to be adsorbed frequently dictates the chemical nature of the adsorbent. For example, carbonaceous adsorbents are often used to selectively remove organic compounds from liquid, gaseous, or vapor media. Silica and alumina based adsorbents are employed to selectively adsorb polar adsorbates such as water, ammonia, and the like from similar media.
The efficacy of an adsorbent for a particular application is usually determined by the adsorption capacity and selectivity of the adsorbent for the adsorbate in question. The adsorption capacity may be measured per unit mass or per unit volume of the adsorbent. In general, the higher the adsorption capacity and selectivity of an adsorbent for a particular adsorbate, the more useful it is, since less of the adsorbent has to be used to effect the same removal of the adsorbate.
Carbonaceous materials, such as activated carbon, carbon black, and the like, represent an important class of adsorbents which are used in many fields such as separation, purification, and waste treatment, among others. Because of their widespread use, any method for improving the adsorption properties of carbonaceous adsorbents for a particular adsorbate can have a large impact on the efficacy and economy of the processes utilizing them. Therefore, attempts have been made in the past to modify the surface chemistry of carbonaceous adsorbents. The methods employed for their modification can be broadly classified into physical and chemical means. In surface modification by physical means, a species is deposited on the surface of the carbonaceous adsorbent to form a layer which then changes its adsorption properties. However, such modification techniques have limited utility because the deposited layer is easily removed. In surface modification by chemical means, the modifying species is attached to the carbon surface by a chemical bonding mechanism.
The characteristics of the adsorption isotherm, representing the relationship between the extent of adsorption and adsorbate concentration or adsorbate partial pressure at a fixed temperature, is also of importance. As described by Sircar et al. in “Activated Carbon for Gas Separation and Storage,” Carbon, Vol. 34, No. 1, pp. 1-12 (1996), the characteristics of the preferred adsorption isotherm will depend on the separation process being employed. For example, in cases where adsorbent regeneration is effected by a pressure swing, the preferred adsorbent is one with a moderate affinity for the adsorbate. When the adsorbate is strongly adsorbed, that is, when it has a strong affinity for the adsorbent, regeneration becomes difficult and energy intensive. On the other hand, when the adsorbent exhibits a weak affinity for the adsorbate, it has a small adsorption capacity at low adsorbent partial pressures and, hence, the adsorption mass transfer zone becomes very long. Thus, the availability of a method for altering the affinity of an adsorbent for an adsorbate is advantageous.
Thus, any method for increasing the adsorption capacity and/or modifying the adsorption affinity of the adsorbent enhances its usefulness in adsorption applications. As already noted, chemical modification can be used to alter the adsorptive properties of carbonaceous adsorbents. The range of chemical species which can be attached, however, is limited.
Bansal, Donnet and Stoeckli (in Chapter 5 of Active Carbon, Marcel Dekker, Inc., 1988) have reviewed different techniques of carbon surface modification. Physical impregnation methods are described, as are methods that rely on chemical reactions with various species to modify the surface of the carbon. Some of the chemical surface modification techniques described by Bansal et al. are oxidation, halogenation, sulfonation, and ammoniation. Several of these techniques require treatment of the carbon at elevated temperatures. Another technique involving oxidation of the carbon with HNO3 in the presence of a catalyst, has been described by Sircar and Golden (U.S. Pat. No. 4,702,749). However, these techniques have certain disadvantages apparent to those familiar with the field.
In view of the limited utility of chemical means of modifying the surfaces of carbonaceous adsorbents, there still is a need to develop modified carbon adsorbents, preferably with improved adsorption capacity over the unmodified material.